Recharges are available on most stores which accept credit cards for payment, they can usually issue electronic vouchers with the same machine, or you can buy cards at newsstands (they are not so common anymore, the physical ones).
BRL 3.00 or 5.00 recharges are valid for 1 day.
BRL 8.00 recharges are valid for 3 days.
BRL 12.00 or BRL 13.00 recharges are valid for 15 days.
BRL 18.00 recharges are valid for 25 days.
BRL 25.00 or more recharges are valid for 30 days.
You can check the balance sending a SMS with SALDO to 8011 you'll get an SMS with the balance.